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ST300 to TH350 Swap

Hi guys, relative newcomer here & I am doing some some long awaited upgrades to my 68 custom convertible . I am losing the 2.56 peg leg in favor a 3.08 posi(8.2 chevy) and retiring the ST 300 in favor of a TH 350 with a shift kit & a slightly looser converter. These 2 upgrades should add a little more zip to the step. M questions are 1) do I need to change my crossmember ? 2) Any ideas on how to swap linkage from my 2 spd to a 3 spd ? 3) Do my tranny coolant lines need to be swapped as I was told the different trannys flow is opposite ? Thanks in advance for your help

300 and 350 are the same length. the 300 is prndl, the 350 is prndll, so no l1. the trans lines don't care where they flow. I don't know about the rear change. might be the same which would use the same driveshaft. you need to check.

Cross member stays as is, 350 should bolt up the same.
I do not know about the 8.2 Chev rear for sure - strong possibility there is no change but measure twice.
Linkage - will bolt right up, you will not see a low position but it is still there. I've got 2 cars with this 2 sp to 3 sp conversion and they both will go into low no prblem.
You did not ask about the kickdown - on the older cars that is a challenge to hook up, totally do-able but takes some thinking.

I'm told I need a bracket that mounts on the manifold and a cable. Also going to need new speedo gear, anybody know who sells one?

The THM350 uses a kick down cable Buick used a cable that went from the gas pedal to the transmission. Chevy used a cable from the carburetor to the transmission. You haven't mentioned what car you are working on.

You can get speedo gears from a number of sources. You'll need to know the rear gear (3.08), the rear tire size (?), and the number of teeth on the transmission DRIVE gear. Then use the formula to figure out what DRIVEN gear you need.